The present invention relates to an electroacoustic transduction film used for an acoustic device such as a speaker or a microphone.
Recently, a flexible display using a flexible substrate of plastic or the like has been studied.
As a substrate of such a flexible display, for example, a flexible display substrate in which a gas barrier layer and a transparent conductive layer are laminated on a transparent plastic film is disclosed in JP2000-338901A.
The flexible display has superiority in lightweight properties, thinness, flexibility, and the like compared to a display of the related art using a glass substrate, and thus is able to be provided on a curved surface of a cylinder or the like. In addition, the flexible display is able to be contained by being rolled up, and thus even when a large-sized screen is used, portability is not impaired, and the flexible display has been attracting attention as a display for displaying advertisement, or a display device such as personal digital assistance (PDA).
When such a flexible display is used as a sound generating device with an image display device which concurrently reproduces an image and a sound as with a television receiver or the like, a speaker which is an acoustic device for generating a sound is necessary.
Here, as the shape of a speaker of the related art, a so-called cone shape such as a funnel-like shape, a spherical dome-like shape, and the like are generally used. However, when such a speaker is embedded in the flexible display described above, lightweight properties and flexibility which are the advantages of the flexible display may be impaired. In addition, when the speaker is attached to the outside of the flexible display, it is difficult to carry the flexible display and to dispose the flexible display on a curved wall, and thus the aesthetic properties of the flexible display may also be impaired.
Among them, as a speaker which is able to be integrated with the flexible display without impairing the lightweight properties or the flexibility, a speaker which is able to adopt a sheet-like piezoelectric film having flexibility is disclosed, for example, in JP2008-294493A.
The piezoelectric film is obtained by performing polarization processing with respect to a monoaxially stretched film of polyvinylidene fluoride (PVDF) at a high voltage, and thus has stretching and contracting properties according to the applied voltage.
In order to adopt the piezoelectric film as the speaker, it is necessary that a stretching and contracting movement is converted into a bending movement of a film surface according to a film surface. This conversion from the stretching and contracting, movement into the bending movement is attained by holding the piezoelectric film in a bent state, and thus the piezoelectric film is able to function as a speaker.
However, it is known that the lowest resonance frequency f0 of a vibration plate for a speaker is denoted by the following expression. Here, s represents the rigidity of a vibration system, and in represents a mass.
Lowest Resonance Frequency:
      f    0    =            1              2        ⁢        π              ⁢                  s        m            
At this time, the mechanical rigidity s decreases as the bending degree of the piezoelectric film, that is, the curvature radius of a bending portion becomes larger, and thus the lowest resonance frequency f0 decreases. That is, the acoustic quality of the speaker (a sound volume, and frequency properties) is changed according to the curvature radius of the piezoelectric film.
In order to solve this problem, in JP2008-294493A, a sensor measuring the bending degree of the piezoelectric film is included, and an amplitude is corrected by increasing and decreasing the amplitude for each frequency band of a sound signal by a predetermined amount according to the bending degree of the piezoelectric film, and thus a stable sound is able to be output.
However, when the speaker formed of the piezoelectric film is integrated, the flexible display having a rectangular shape in a plan view is held in a state of being loosely bent like a newspaper or a magazine as a portable display, and the screen display is used by being vertically and horizontally switched, it is preferable that an image display surface is able to be bent not only in a vertical direction but also in a horizontal direction.
However, the monoaxially stretched piezoelectric film formed of PVDF has in-plane anisotropy in the piezoelectric properties thereof, and thus the acoustic quality is considerably changed according to a bending direction even at the same curvature.
Further, PVDF has a smaller loss tangent than a general vibration plate for a speaker such as cone paper, and thus a strong resonance is easily generated, and strong undulation is included in frequency properties. Accordingly, the changed amount of the acoustic quality at the time that the lowest resonance frequency f0 is changed according to a change in the curvature increases.
Thus, in an acoustic quality correction means disclosed in JP2008-294493A, it is difficult to reproduce a stable sound due to the PVDF-specific problem.
On the other hand, as a sheet-like piezoelectric material which has no in-plane anisotropy in the piezoelectric properties and has flexibility, a polymer composite piezoelectric body is included in which piezoelectric ceramics is dispersed in a polymer matrix.
In a case of the polymer composite piezoelectric body, the piezoelectric ceramics is rigid but the polymer matrix is flexible, and thus energy may be absorbed before the vibration of the piezoelectric ceramics is entirely transmitted. This is the transmission efficiency of the dynamic vibration energy, and it is necessary to harden the polymer composite piezoelectric body in order to improve the transmission efficiency, and thus it is necessary that at least 40% to 50% of the piezoelectric ceramics is put into the matrix in a volume fraction.
For example, In Toyoki KITAYAMA, 1971, Proceedings of the Institute of Electronics, Information and Communication Engineers General Conference, 366 (1971), it is disclosed that a polymer composite piezoelectric body in which a powder of PZT ceramics which is a piezoelectric body is mixed with PVDF by using solvent casting or hot kneading makes the flexibility of PVDF and high piezoelectric properties of the PZT ceramics compatible to a certain degree.
However, when the ratio of the PZT ceramics increases in order to increase the piezoelectric properties, that is, the transmission efficiency, a mechanical defect that the polymer composite piezoelectric body becomes hard and brittle is generated.
In order to solve this problem, for example, in Seiichi SHIRAI, Hiroaki NOMURA, Toshio OGA, Takeshi YAMADA, and Nobuki OGUCHI, The Institute of Electronics, Information and Communication Engineers Technical Report, Information Theory, 24, 15(1980), it is disclosed that fluorine-containing rubber is added to PVDF, and thus flexibility is maintained.